Various toners are being used as the electrophotographic toner, however, almost all are amorphous toners prepared by the so-called grinding method and a very small number of toners are spherical toners prepared by the polymerization method.
In recent years, the toner is required to satisfy various demands so as to improve the image quality or save energy of a copying machine or printer. For example, the toner is keenly required to have a small particle size for improving the image quality, however, the amorphous toner prepared by the grinding method cannot be free of conspicuous reduction in the fluidity as a result of formation into small particles. Further, the colorant comes out on the surface of a toner particle and accordingly, the control of electrostatic charge is troublesome. Furthermore, in order to reduce the demand power of a copying machine or printer, the toner must have both the capability of fixing at low temperatures and the hot-offset resistance.
Almost all toners (in particular, a negatively polar toner comprising a styrene-acrylic resin as the binder) use a CCA (charge control agent), however, the CCA contains a heavy metal and may pollute the environment and therefore, it is desired, if possible, not to use the CCA.
The electrophotographic toners known up to the present time in the field of conventional techniques such as the grinding method or polymerization method have not succeeded in satisfying all these requirements.
The process for preparing a toner by the phase inversion emulsification method is a new technique of which basic technique is first disclosed in JP-A-5-66600 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). JP-A-5-66600 describes a negatively charging toner comprising an anionic self-emulsifiable binder resin having encapsulated therein a colorant and the preparation process thereof, in which the particle size distribution and the triboelectricity are mainly studied but the fixing property is not specifically compared and studied. The binder resin used here is a styrene-acrylic resin having a weight-average molecular weight of from 35,000 to 60,000 and in the light of the technical structure of the present invention, the styrene-acrylic resin obtained according to JP-A-5-66600 comprises less than 4% of the components having a molecular weight in terms of polystyrene by GPC of 200,000 or more and has a Mw/Mn ratio of about 3.
To speak roughly, the preparation process of the toner by the phase inversion emulsification method is a technique of dispersing an organic solvent solution of a resin containing a hydrophilic group and having a self-emulsifiability, a colorant and the like in an aqueous medium and forming a spherical particle having encapsulated therein a colorant.
This process is advantageous in that the particles can be easily prepared without consuming huge energy such as grinding and at the same time, small size particles can be very easily prepared. Further, particles having a sharp grain size distribution can also be prepared. Furthermore, since a surface active agent or protective colloid is not substantially used, the cleaning operation can be simple and the problem of environmental stability of the charging is relatively lightened, as compared with the process using a dispersion stabilizer such as a polymerization toner.
However, in the preparation process of the toner by the conventional phase inversion emulsification method, an organic solvent solution of resin is used and a cross-linked resin cannot be used substantially. Further, even if the resin used is not crosslinked, when the resin used contains less than 4% of the components having a molecular weight in terms of polystyrene by GPC of 200,000 or more and has a Mw/Mn ratio of about 3 as described above, it is difficult to achieve satisfactory fixing properties.
Furthermore, the toner prepared by the phase inversion emulsification method using no surface active agent nor protective colloid is disadvantageous in that since the toner particle contains a polar group having hygroscopicity, the environmental stability (the degree of deflection in the triboelectricity of toner upon changing of the temperature or humidity) is insufficient and also it is considered to be ascribable to the spherical shape of the toner particle, the charge rising at the triboelectric charging is deficient.
The object to be attained by the present invention is to remarkably improve the heated roll fixing property by specifying the molecular weight distribution of the binder resin in the light of the construction of the binder resin used in JP-A-5-66600.
The ultimate object to be attained by the present invention in an optimized embodiment is to provide a toner which (1) is excellent in the heated roll fixing property, (2) has good fluidity even when the particle is formed to have a small size and (3) exhibits good triboelectricity even without using a CCA, more specifically, to provide a spherical toner capable of fixing with a heated roll over a wide fixing temperature range and having excellent triboelectricity while improving the thermal fixing property and the environmental stability of charging which are the problems most earnestly desired to solve in the conventional phase inversion emulsification method.